When I was commuting daily I had one on each side but then again I rode on the wrong side of the road a lot...
I am thinking to go with a helmet mount and one mirror on the handlebars I use barend shifters usually so the bar end mirror is not an option..
I also found that I was adjusting the mirrors a lot with the flexible wire then I went to clamp mounts and it was much better.
The helmet or glasses mount is my next purchase...